This invention relates to a method for the production of a decontaminating liquid for removing radioactive substance adhering to or deposited on the surface of a radioactive metal waste which is generated from nuclear power generation facilities and nuclear fuel recycling facilities.
Radioactive oxide layers form inside pipes, machines and vessels, fuel assemblies, etc. which have been used in nuclear power generation facilities and nuclear fuel recycling facilities. They constitute themselves a cause for increasing the surface dose rate. Removal of these oxide layers from the aforementioned mechanical articles, i.e. decontamination of the mechanical articles, therefore, is indispensable for the purpose of decreasing the possibility of attendants being exposed to radiation as during the course of a periodic inspection. Generally in this removal, the oxide layers formed on the surface of metal materials, corrosion products resulting from dissolution, adhesion, or sedimentation of matrix metal materials, and nuclear fission products are required to be shed into a liquid. Various methods which are now available for the contamination of the nature just mentioned may be broadly divided into chemical methods of decontamination, physical methods of decontamination inclusive of mechanical methods, and electrochemical methods of decontamination. The chemical methods resort to use of a decontaminating agent formed by blending an oxidation-reduction agent, a complexing agent, and an inhibitor with due respect to the characteristic properties of the oxide layers. These methods are advantageous in terms of the speed of solution of oxide layers but entail the disadvantage of giving rise to a large volume of a secondary waste.
As means of overcoming the drawback, the specification of U.S. Pat. No. 4,217,192, for example, discloses a process for the decontamination of metals contaminated by radiation in the nuclear power industry and a system therefore, which make use of chemical etching.
This is a chemical etching process utilizing an oxidation-reduction effect, which process comprises immersing a metal article subjected to contamination and a pair of electrodes intended for regeneration of an exhausted electrolyte together in an electrolyte (decontaminating liquid) formed from a nitric acid solution containing trivalent cerium ion and tetravalent cerium ion and held in a vessel thereby depriving the metal article of contaminating radiation, cleaning the decontaminated metal article, and removing the cleaned metal article out of the electrolyte.
In accordance with the process described above, however, the electric current for the formation of the tetravelent cerium ion is defined as falling in the range of 0.5 to 1.5 A and a decontaminating liquid consisting of 0.01 to 0.3 mol/liter of tetravalent cerium ion and 1 to 8 mol/liter of nitric acid is produced but the conditions of electrolysis for the formation of the tetravalent cerium ion are not clearly indicated. The inventors have had it experimentally ascertained that when the electric current falls in the aforementioned range of 0.5 to 1.5 A, and a different material for the cathode is used, a reaction causing reduction of tetravalent cerium ion to trivalent cerium ion occurs in the place of a reaction producing hydrogen at the cathode and, as a result, the electric current forming efficiency of the tetravalent cerium ion is degraded in efficiency. Similarly, in the aforementioned range of 0.5 to 1.5 A, and when a different material for the anode is used, a reaction causing oxidation of trivalent cerium ion to tetravalent cerium ion occurs in the place of a reaction producing oxygen at the anode; as a result, the elecric current forming the tetravelent cerium ion is degraded in efficiency. The decontamination step does not embrace any treatment for the separation of insoluble substances sedimented or suspended in the decontaminating liquid which has been spent in contamination. In the electrolyte (decontaminating liquid), therefore, the debris arising from the etching work gradually collects in the vessel to form a slurry or suspension and grows worse with the consumption of the electrolyte, preventing the electrolyte from filling its part satisfactorily and shortening the service life of the electrolyte.
Further, since this process consists in a batchwise operation and not in a continuous operation, it requires time and labor in preparation for each batch of operation and inevitably lengthens the total period of exposure of workers to radiation. Besides, since the process requires use of strong nitric acid and elevated temperatures, it entails generation of sulfur dioxide gas and nitric acid mist and consequent deterioration of the work environment. In the circumstances, the desirability of developing a method for the production of a decontaminating liquid which enjoys high current efficiency, permits continuous regeneration, and avoids heavily yielding secondary waste has been finding growing recognition.